MEMS enables electrically trimmable passive resistor

PORTLAND, Ore. -- The inner workings of the world's first analog integrated circuit to incorporate electrically adjustable passive resistors will be described Wednesday at the International Solid State Circuits Conference. ISSCC is being held Feb 3-8 in San Francisco. Microbridge Technologies Inc. will detail a trimmable Wheatstone bridge for piezoelectric sensors, made adjustable by virtue of its patented "rejustor" technology based on microelectromechanical systems (MEMS).

"The chip is designed as an adjustable conditioner for piezoelectric bridge sensors," said Les Landsberger, chief technology officer and founder of Microbridge (Sunnyvale, Calif.).

The Wheatstone bridge for piezoelectric sensors, which provides analog calibration, temperature compensation and amplification, was enabled by a proprietary electrically readjustable resistor (or rejustor) technology. Microbridge has perfected the rejustor over the last several years by designing application-specific discrete devices for its customers. Currently, four chip makers are negotiating licenses to integrate the rejustor technology onto their own analog chips.

However, the trimmable Wheatstone bridge for piezoelectric sensors the company will describe at ISSCC marks the first time a rejustor has been integrated onto a CMOS chip alongside other circuitry. Microbridge says more are in the pipeline.

"We already have the technology ready for many other designs and anticipate introducing many other standard IC products over the next few years," said Bob Frostholm, vice president of marketing, strategic alliances and business development.

The rejustor is designed as an inexpensive alternative to the expensive, laser-trimmed resistors and intricate active circuitry now used to compensate for inaccuracies introduced while manufacturing, assembling and packaging analog ICs. The trimmable Wheatstone bridge for piezoelectric sensors can compensate for variation in both offset and sensitivity, as well as for temperature-induced offset and sensitivity drift.

"No longer do you need to be an expert analog engineer to design and build expert analog circuits," said Frostholm. "With the rejustor technology, you can design very inexpensive circuits that nevertheless compete favorably with very expensive analog devices---from reference to converters to communications amplifiers--achieving very high-precision devices from relatively nonprecise designs."

The company says the technique will also cut the cost of analog chip designs, by virtue of the smaller die area rejustors consume compared with other high-precision resistor technologies.

"Ordinarily you need a lot of silicon real estate to produce a very precise resistor, but by building in a rejustor you can use very small, imprecise resistors in your design and just trim the rejustor to achieve almost any degree of precision," Frostholm said.

The unique aspect of construction is that rejustors are made from polysilicon, which can be calibrated to an accuracy of 0.1 percent.

"We think our high precision will shake up the analog IC community," said Frostholm. "All of their designs are based around accuracy of resistors being no better than 5 percent, which has forced many EEs to resort to either expensive laser-trimmed thin-film resistors or to add active circuitry compensation."

Unlike digital potentiometers, rejustors are purely passive devices, requiring only a minimal amount of current to adjust their value. The basic principle of their operation is that polysilicon changes its resistance when heated and, depending on the annealing method used, can be adjusted to a relatively wide range of resistance values.